472 Prof. Challis's Explanations of Phenomena of Light 



to a result of the composition of undulations. Having treated 

 of this subject expressly in an article on " The Theory of the 

 Composition of Colours/' contained in the Philosophical Maga- 

 zine for November 1856, I shall at present simply refer to the 

 arguments there adduced, as giving evidence that the theory of 

 undulations accounts for many phenomena of that class. The 

 subject, however, is open to further investigation both theoreti- 

 cally and experimentally. 



I have now completed the consideration of those phenomena 

 of light the theoretical explanations of which depend only on pro- 

 perties of the assumed sethereal medium. The facts and laws which 

 have received explanations have been indicated by italics, in order 

 that their number, variety, and speciality may be the more 

 readily perceived. When it is considered that all these expla- 

 nations have been evolved by mathematics alone from the single 

 hypothesis that light is generated and transmitted in a fluid 

 medium the pressure of which varies as its density, the evidence 

 for the actual existence of such a medium would seem to be as 

 complete as any to be met with in the whole range of applied 

 mathematics. All phenomena that have been thought to be 

 explained on any other hypothesis are capable of explanation on 

 this, together with a great many more. I cannot, therefore, 

 imagine any reason for giving preference to" the theory which 

 attributes the phenomena of light to the oscillations of the dis- 

 crete atoms of a hypothetically constituted medium, especially 

 as I have also shown, by arguments unanswered and unanswer- 

 able, that that theory is beset with insuperable difficulties. 



I feel called upon to take this opportunity for adverting again 

 to the reference made by Professor Tyndall to my views on La- 

 place's theory of the velocity of sound, because, when I wrote 

 the reply at the end of my communication to the Philosophical 

 Magazine of last August, I had not had before me the very 

 remarkable and significant experimental results contained in his 

 paper " On the Absorption and Radiation of Heat by Gaseous 

 Matter " (Phil. Trans, part 1, 1862, and Phil. Mag. for October 

 1862). These results have a direct bearing on the question at 

 issue, but not such as Professor Tyndall supposes them to have. 

 As far as regards my discussion with Professor Stokes, the prin- 

 cipal physical point involved was, the sense in which I used the 

 term " radiation/' which he said was different from that in 

 which he usually understood it. This might well be the case, 

 because the fact which the term was intended to express was 

 looked at from a theoretical point of view, and did not at that 

 time admit of being substantiated for want of experiments. 

 These have now been supplied by Prof. Tyndall, as a few words 

 will suffice to show. I had long ago conceived that the expla- 



